1. Field of the Invention
The present invention relates to an air intake manifold for an internal combustion engine, particularly of the type comprising at least one combustion chamber and at least one cylinder in which a piston is given a reciprocating movement, the combustion chamber being supplied with air and with fuel by the opening of at least one intake valve which selectively closes an intake duct in a cylinder head of the engine.
2. Description of the Related Art
To supply such an engine with air, it is known practice for use to be made of an air intake manifold comprising:
an upstream plenum, common to all the downstream air ducts feeding the combustion chambers of the engine and supplied with air through an air inlet orifice connected to a device for controlling the intake of air to the engine, such as a throttle body, mounted upstream of said inlet orifice, and,
for each combustion chamber of the engine, at least one of the downstream air ducts connecting a respective outlet orifice of the plenum to a corresponding supply orifice of the corresponding combustion chamber of the engine, the supply orifice being formed in at least one flange that fixes the manifold to the cylinder head of the engine, the plenum and each of the air ducts at least being made of synthetic material which is generally a technicalxe2x80x94grade plastic.
In these known intake manifolds, essentially made of plastic, the air ducts generally have a passage cross section which is constant in terms of shape and/or in terms of cross-sectional area, or which have a progressive change in shape, changing progressively from a circular cross section to a rectangular or square cross section with rounded corners, or vice versa, and/or a passage cross section which decreases very progressively and very slightly from the upstream end downstream, to form convergent ducts.
Whatever the engine, there are speed and load combinations for which it is possible to find a corresponding position of the throttle valve that controls the stream of air let into the engine, during the lift of an intake valve, upon the opening by the latter of the corresponding intake duct, in the intake phase into a corresponding combustion chamber of the engine, and an intake valve lift amplitude for which the air flow conditions around the intake valve are sonic conditions, at the same time as the surfaces wetted by the air flow over this intake valve and over the seat of this valve are such that brief strident whistling occurs, of high acoustic intensity.
Given that the onset of the whistling is brought about by a specific and tailored ratio between the throughput cross section of the throttle body and that of the intake cross section, the noise is manifested in a sustained way on an engine turning over during a rotational movement of said throttle valvexe2x80x94typically during an opening at low engine rotational speed from light load toward heavy load.
When the intake manifold as defined hereinabove is made of a metal casting, such as an aluminum casting, its mass is high enough for the whistling to be deadened sufficiently to the point where it is not deemed to be troublesome with respect to the criteria customarily applied to the optimizing of a vehicle. By contrast, in the case of air intake manifolds made essentially of plastic materials, it is found that this strident whistling is not deadened and may be audible from outside, when not concealed by other running noises of the engine and of its environment.
The problem underlying the invention is that of overcoming this particularly troublesome disadvantage of air intake manifolds of the type defined hereinabove, essentially made of plastic and, more generally, of proposing means making it possible to make air intake manifolds quieter.
To this end, the intake manifold according to the invention, of the type explained hereinabove, is characterized in that, for each combustion chamber of the engine, at least one corresponding downstream air duct has at least one sudden enlargement of its passage cross section, followed immediately in the downstream direction by a duct portion having a progressive reduction in cross section in the downstream direction, ending upstream of the flange for fixing to the cylinder head of the engine.
The sudden enlargement of the air duct thus creates a lag interface in the transmission of the pressure gradient signal, caused in the region of the throttle body, because of the sudden variation in the density of the air in the sudden enlargement, and the duct portion with the progressive reduction in cross section which follows on from the sudden enlargement constitutes a zone in which the air stream reaccelerates, at least partially compensating for the pressure drop caused by the sudden enlargement, and allowing energy recovery similar to that which can be obtained by a Venturi effect.
Thus, the noise is not eliminated but offset, with respect to the conditions suited to whistling at the intake valve and its seat, by virtue of a lag in the transmission of the pressure wave, which is introduced into the air duct.
The sudden enlargement by sudden increase in the passage cross section of the air duct may be ensured on at least two opposite sides of this air duct or along the entire periphery thereof, particularly when its cross section is circular or oblong, it being possible for the sudden enlargement and the duct portion with the progressive reduction in cross section to be of revolution about the longitudinal axis of the duct. However, it is also possible for the sudden increase in cross section to be ensured on just one side of the longitudinal axis of the air duct, something which is advantageous when each air duct is produced by joining together, for example by thermal fusion or by ultrasonic or vibration welding, two half-shells each molded as a single piece, because the sudden enlargement and the duct portion with the progressive reduction in cross section can be exhibited by at least one of the two half-shells, on which this enlargement and this portion with progressive reduction of cross section are produced by the molding of this half-shell.
Advantageously also, the ratio of the cross section of the air duct at the sudden enlargement to the cross section of the air duct immediately upstream of said sudden enlargement is of the order of 2.
Furthermore, if the cross section of the air ducts is substantially quadrilateral, it is advantageous for the duct portion with a progressive reduction in cross section to extend over a length, along the longitudinal axis of the air duct, of between about 2.5 and about 3 times the height of the sudden enlargement, which height is measured substantially at right angles to said longitudinal axis of the air duct at said sudden enlargement, if, indeed, the width of the air duct at right angles to its height and its length is substantially constant.
When, in the known way, each air duct has, upstream of the corresponding supply orifice in the flange for fixing to the cylinder head, a housing for a fuel injector injecting this fuel to the supply orifice, it is also advantageous for the duct portion with a progressive reduction in cross section to have its downstream end situated upstream of said housing for the injector.
In general, it is advantageous for the sudden enlargement and the duct portion with a progressive reduction in cross section to be situated substantially in the downstream half of the corresponding air duct.
The sudden enlargement and the duct portion with a progressive reduction in cross section may be situated in a substantially straight part of the air duct in which case, according to a simple and economical structure of the manifold, the duct portion with a progressive reduction in cross section is advantageously at least partially delimited by at least one inclined plane if the air duct has a rectangular cross section, or by at least one portion of a cone frustum, if the air duct has a circular or oblong cross section.
By contrast, if the sudden enlargement and the duct portion with progressive reduction in cross section are situated in a curved part of the air duct it is preferable for the duct portion with a progressive reduction in cross section to be at least partially delimited by a curved surface with the concave side facing in the same direction as the curved part of the air duct.